Place the Hilsa fish pieces on a plate. Sprinkle salt and 1/4 tsp turmeric on both sides. Rub gently so the spices coat the fish evenly. This step ensures that the fish absorbs flavour before frying.
Heat mustard oil in a deep pan until it starts to smoke. Carefully place the fish pieces and sauté on high heat for 1 minute on each side. Remove and drain on a banana leaf-lined plate to retain aroma and prevent sticking.
Add onion seeds to the remaining oil and sauté briefly. Toss in the green chillies, then mix in the remaining turmeric, red chilli powder, and mustard paste. Cook for 1 minute, add 1 cup of water and salt, and let it boil gently for 2-3 minutes.
Pour milk into the pan and stir well. Simmer until the sauce thickens slightly, forming a creamy base that will coat the fish perfectly.
Gently place the fried fish pieces into the mustard sauce. Spoon the sauce over the fish carefully to avoid breaking it. Cook on low heat for 5-6 minutes until the fish is tender and fully coated with the sauce.
Transfer the fish to a serving bowl. Garnish with slit green chillies. Serve immediately with steamed rice for an authentic festive Bengali meal.